Tachycardia is a cardiac condition whose symptom is a fast heartbeat rate. There are different types of classifications of tachycardia (also referred to in the literature as “tachyarrhythmia”) including: (1) tachyarrhythmias that are pathological in origin, including ventricular fibrillation (VF), ventricular tachycardia (VT) and supra-ventricular tachycardia (SVT), covering atrial tachycardia, atrial flutter and atrial fibrillation (FA); and (2) sinusal tachycardia (“ST”) that are physiological in origin.
The diagnosis and the classification of tachycardia are generally performed in a known manner. Suitable examples are described e.g., in EP-A-0 626 182 and its corresponding U.S. Pat. No. 5,462,060 and EP-A-0 838 235 and corresponding U.S. Pat. No. 5,868,793, all commonly assigned herewith to Ela Médical, S. A. Montrouge, France. U.S. Pat. Nos. 5,462,060 and 5,868,793 are incorporated herein by reference in their entirety. The diagnosis and classification described therein are based upon selected criteria such as the ventricular frequency, the stability of the ventricular intervals (RR intervals), an analysis of atrio-ventricular association (such as is revealed by the stability of the PR interval) and the mode of starting of the tachycardia (e.g., a presence of an abrupt acceleration and the cavity of origin of the tachycardia (ventricular or atrial)).
The algorithms used to perform the diagnosis and classification as described in the above mentioned patents are particularly effective to discriminate situations where it is necessary to deliver a ventricular therapy (true VT) from those situations where it is not necessary to do so (ST or SVT, because in their latter cases the tachycardia is of atrial origin and an atrial therapy can be considered).
Clinical implementation of these algorithms and the resultant decision to apply a particular therapy, however, has revealed a difficulty during the time immediately following the application of a therapy. Indeed, after the application of a shock or an ATP stimulation therapy, the algorithm is reiterated to determine whether a therapy must be again applied or, on the contrary, the therapy already applied caused the tachyarrhythmia to disappear, as was the purpose of the delivery of this therapy. The clinical study indeed revealed that certain cases of post-therapy ST were wrongly interpreted by the algorithm as VT, and thus unnecessarily caused the repetition of the therapy.
However, it is desirable to avoid applying a therapy when it is not necessary because, among other things, the unjustified application of a therapy can have noxious (adverse)consequences on the evolution of the heartbeat rate of the patient, the therapy, in particular in the form of a cardioversion or defibrillation shock, is painful for the patient and must thus be limited to cases of necessity, and a repeated therapy will worry the patient and the clinician, who will believe wrongly that the initial therapy was not effective whereas the repetition of the therapy is the result only of an incorrect interpretation of the situation by the algorithm.